The present invention relates generally to externally illuminated display devices and more particularly to such devices of the type wherein information is displayed on a surface which is part of a liquid crystal display.
In contrast to such active display devices as light emitting diodes and cathode ray tubes which generate their own light, liquid crystals displays are passive, generating no light of their own. Information is displayed on liquid crystal displays in the form of images which are of various levels of darkness or apparent reflectivity and are rendered visible by light which impinges on the display surface from an artificial or natural external source.
The particular type of liquid crystal display with which the present invention is concerned is commonly referred to as a reflective mode dynamic scattering liquid crystal display. This type of display must be illuminated from its front and, due to the scattering nature of the activated liquid crystal, the angle of reflection must be close to the viewing angle for adequate brightness to the viewer from the scattering regions. Available arrangements for illuminating an upright reflective mode dynamic scattering liquid crystal display are characterized by conventional projection optics or direct illumination by light sources which extend in front of the display. They tend to obstruct the full field of view, are bulky and have no provision for illuminating the upright display with ambient light.
Accordingly, it is the principal object of the present invention to provide means for directing light at a reflective mode dynamic scattering liquid crystal display which lends itself to operation both with an ambient and an artificial light source.
A related object of the invention is to provide such a means which is compact, inexpensive and does not interfere with the field of view.
These and other objects of the invention are accomplished by a display apparatus having an upright, reflective mode dynamic scattering liquid crystal display and illuminated from an overhead source, wherein a Fresnel reflector is mounted in front and below the display and extends laterally therefrom. The reflector is provided with a plurality of facets disposed in a plurality of planes, each of them disposed at an acute included angle with the display surface of the liquid crystal display, so that light from the overhead source impinging on the reflector is reflected thereby at the display surface. The foregoing arrangement is readily adaptable for directing light at a desired angle at the liquid crystal display either from an artificial light source mounted directly above the reflector, or from ambient light impinging from above onto the reflector. The facets of the reflector may be either parallel, so that light from above is reflected in parallel rays at the display, or they may be disposed at progressively diminishing included angles with the surface of the display at progressively greater distance away therefrom, so as to cause light projected at the display to converge at a focal point which is behind the display surface.